Method for combusting fuel in a fired heater

ABSTRACT

A method for combusting fuel in a fired heater having one or more burners, the method comprising: (a) combusting at least a portion of the fuel in the presence of an oxidizer in the burners, generating heat and producing a flue gas; and (b) recycling at least a portion of the flue gas to the burners; wherein step (a) and step (b) are conducted in a manner such that the combusting takes place in a combustion regime represented by  
           4   ,   500     ≥     7554.8   -     933.72      x     +     64.960        x   2       +     .47705      y     -     .55680      z     -     1579.2   w       ≥     2   ,   500       ;                 
 
     wherein w represents a mole fraction of oxygen in the oxidizer, x represents a recycle ratio of the flue as measured in moles of flue gas/mole of oxidizer; wherein y represents a temperature of the flue gas in Fahrenheit recycled to the burners; and wherein z represents a heating value of the fuel in Btu/scf LHV.

FIELD OF INVENTION

[0001] The present invention relates to a method for combusting fuel andmore particularly to a method for combusting fuel in fired heaters inthe presence of substantially pure oxygen and recycled flue gas.

BACKGROUND OF THE INVENTION

[0002] Current and future environmental regulations necessitate designand operation changes to existing and new fired heaters and boilers usedin many industrial processes, including the refining of petroleum.Conventional fired heaters, as traditionally operated, produce fluegases comprising water vapor (H₂O), carbon dioxide (CO₂), unburned fuel,nitrogen oxides (NO_(x)), sulfur oxides (SO_(x)) and nitrogen (N₂). Ofthese exhaust gases, carbon monoxide (CO), unburned fuel, nitrogenoxides (NO_(x)), and sulfur oxides (SO_(x)) are objectionableenvironmental pollutants and/or health hazards.

[0003] More particularly, CO is an odorless toxic gas that causes avariety of physical ailments, including headaches, nausea,unconsciousness and, ultimately, death, upon prolonged exposure. NO_(x),which comprises NO, N₂O₃ and NO₃, reacts with hydrocarbons in thepresence of oxygen and sunlight to form a photochemical smogcontributing to the “Green House” effect in the Earth's atmosphere.Unburned hydrocarbons also contribute to smog and the “Green House”effect in the Earth's atmosphere. SO_(x), which comprises SO₂ or SO₃,produces acid rain and is toxic.

[0004] In light of these harmful pollutants and environmentalregulations, control of the pollutants has become a primary designparameter for fired heaters and other fuel combustion devices. However,heater design has not evolved to the level that satisfies environmentaland health concerns while also satisfying the economic and technicalpracticalities of operating fired heaters.

[0005] Of particular and recent concern in heater design is thereduction of NO_(x), NO_(x) reduction may be accomplished through theelimination of N₂ as an oxidant during combustion processes. However, N₂is the major component of air, which provides the oxygen necessary forthe combustion of hydrocarbons. Consequently, some of these efforts havesuggested using pure oxygen in lieu of air to remove N₂ from thecombustion process.

[0006] Although using pure oxygen greatly reduces the presence of N₂during the combustion process, the use of pure oxygen presentsadditional problems not associated with the use of air to combust fuel.Using pure oxygen in a conventional fired heater results in elevatedradiant section temperatures due to the absence of N₂, which removesheat from the radiant section of fired heaters. Such elevatedtemperatures pose substantial safety and environmental risks including,but not limited to, heater material failure. Additionally, elevatedtemperatures may cause degradation of hydrocarbon process fluids thatare heated in the tubes of the radiant section of the heater resultingin coking and unit downtime for many chemical and refining processes. Ifthe fuel contains nitrogen or nitrogen enters the heater from theambient air, these elevated temperatures also pose the environmentalhazard of converting excessive quantities of the nitrogen to NO_(x).Additionally, the presence of the nitrogen may cause a reduction offired heater efficiency, therefore requiring increased fuel consumptionand increased CO₂ emissions to the atmosphere.

[0007] Consequently, to moderate the radiant section temperature, somepublications have suggested recycling combustion exhaust gases to thecombustion process, alleviating the need for N₂ as a temperaturemoderator. Wilkinson et al., CO ₂ Capture via Oxyfuel Firing:Optimisation of a Retrofit Design Concept for a Refinery Power StationBoiler, First National Conf. on Carbon Sequestration (May 2001),discloses a method for capturing CO₂ from boilers for heating water bycombusting fuel with pure oxygen and a flue gas recycle stream. Althoughthe Wilkinson disclosure advances the art, the Wilkinson boiler islimited to boiler design and does not address critical design andoperational aspects of heating hydrocarbon process fluids while reducingNO_(x) emissions.

[0008] Additionally, the Wilkinson publication and similar efforts havenot been widely commercialized, and attempts to design and commercializesuch efforts have revealed potentially catastrophic problems andchallenges. For example, high temperatures due to rapid oxidation offuels can be catastrophic in the absence of large quantities of nitrogento remove heat from the radiant section of conventional fired heaters.In flue gas recycle designs, changes in fuel heat content or flue gasheat capacity can create dangerous conditions that are not realizeduntil they are fully involved, which may jeopardize safe and economicaloperation of the fired heater.

[0009] We have now found that combusting fuel in a fired heater in thepresence of pure oxygen within a combustion regime represented by${{4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500}},$

[0010] wherein w is the mole percent of oxygen in the oxidizer, x is therecycle ratio of the flue gas as measured in moles of recycle fluegas/mole of oxidizer, y is the temperature (° F.) of the flue gasinjected into the radiant section of the fired heater, and z is theheating value of fuel in the combustion process, as measured in Btu/scfLHV, results in unprecedented safe and environmentally sound operationof a fired heater.

[0011] We have also found that combusting fuel in the presence ofsubstantially pure oxygen while recycling flue gases in a fired heatersuch that the radiant section of the heater operates at a pressuregreater than ambient pressure results in substantially zero NO_(x)produced from ambient air infiltration while maintaining safe andefficient operation.

[0012] We have also found that continuous online monitoring of the fuelcomposition, flue gas temperature and flue gas recycle rate allows forrapid and accurate control of fuel combustion in a fired heater withinthe combustion regime represented by${{4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500}},$

[0013] resulting in unprecedented safe and environmentally desirableoperation of a fired heater.

SUMMARY OF THE INVENTION

[0014] The present invention is directed to a method for combusting fuelin a fired heater having one or more burners, a flue gas stack section,and a flue gas recycle stream. The method comprises the steps ofcombusting at least a portion of the fuel in the presence of an oxidizerin the burners producing a flue gas and recycling at least a portion ofthe flue gas to the burners, wherein the combustion process has acombustion regime represented by${{4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500}};$

[0015] and wherein w is the mole fraction of oxygen in the oxidizer, xis the recycle ratio of the flue gas recycled to the burners, asmeasured in moles of flue gas/mole of oxidizer, y is the temperature inFahrenheit of the flue gas recycled to the burners, and z is the lowerheating value of the fuel in Btu/scf.

[0016] In another embodiment, the present invention is directed to amethod for operating a fired heater having a radiant section wherein themethod comprises the steps of combusting a fuel in the presence ofsubstantially pure oxygen in one or more burners of the fired heater,producing a flue gas and recycling at least a portion of the flue gas tothe one or more burners, wherein the radiant section operates at apressure greater than the ambient pressure.

[0017] In yet another embodiment, the present invention is directed to amethod for monitoring and controlling the combustion of a fired heatercomprising a fuel feed stream, an oxygen feed stream and a recycle feedstream, the method comprising the steps of monitoring the fuel stream,producing data representative of heating value z of the fuel, feeding atleast a portion of the fuel to the fired heater, combusting the fuel inthe presence of the substantially pure oxygen in one or more burners ofthe fired heater producing a flue gas having a temperature y in degreesFarenheit, recycling at least a portion of the flue gas into the oxygenfeed stream producing an oxygen and flue gas mixture, and thereafterfeeding the mixture to the fired heater, wherein the fuel, substantiallypure oxygen and flue gas are fed into the fired heater in stoichiometricamounts to maintain a combustion regime represented by${4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500.}$

[0018] The present invention provides for substantial cost-savingsduring the operation of a fired heater, such as reducing unnecessaryconsumption of expensive oxygen and fuel.

[0019] The present invention provides for substantial safety benefits toperson and property by avoiding dangerous operating conditions, whichcan result in the discharge of toxic pollutants into the atmosphere.

[0020] The present invention provides for substantial safety benefits toperson and property by avoiding dangerous operating conditions that canresult in material failure of the fired heater.

[0021] The present invention also provides for a simple design optionfor the combustion of fuel in an oxygen-only fired heater, facilitatingretrofitting of existing fired heaters utilized throughout industry.

BRIEF DESCRIPTION OF THE DRAWING

[0022]FIG. 1 is an embodiment of a process in accordance with thepresent invention including methods for continuous monitoring andcontrol of the subject invention.

[0023]FIG. 2 is a three dimensional plot in accordance with the presentinvention graphically representing the combustion regime as hereindescribed.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0024] In greater detail, fuels suitable for the subject inventioninclude, but are not limited to, natural gas, refinery fuel gas, or anyliquid or gas fuel suitable for combustion in fired heaters. However,for the purposes of the subject invention, it is preferred to identifysuitable fuels by their heating values, which are typically expressed inBtu/scf for gaseous fuels and Btu/lb for liquid fuels. The gas fuelssuitable for the subject invention will typically have a lower heatingvalue (LHV) from about 100 Btu/scf LHV to about 2,200 Btu/scf LHV.However, it is preferred that the subject invention employs fuels havinga heating value ranging from about 300 Btu/scf LHV to about 1,200Btu/scf LHV, and most preferably from about 300 Btu/scf LHV to about 900Btu/scf LHV for best results. The liquid fuels suitable for the subjectinvention will typically have a heating value from about 17,450 Btu/lbLHV to about 18,200 Btu/lb LHV. However, it is preferred that liquidfuels suitable for the subject invention have a heating value from about18,100 Btu/lb LHV to about 18,170 Btu/lb LHV, for best results.

[0025] The oxidizer suitable for the subject invention can beoxygen-enriched air or substantially pure oxygen. The oxygen-enrichedair may comprise between about 30 to about 94 mole percent oxygen (about0.300 to about 0.940 mole fraction oxygen) with the balance beingnitrogen and possibly other trace elements, including, but not limitedto argon and carbon dioxide. It is preferred that the oxygen-enrichedair comprises at least 40 mole percent oxygen (at least 0.400 molefraction oxygen), more preferably at least 50 mole percent oxygen (atleast 0.500 mole fraction oxygen), and even more preferably at least 80mole percent oxygen (at least 0.800 mole fraction oxygen) with thebalance being nitrogen and possibly other trace elements. Althoughoxygen-enriched air may be used as the oxidizer, it is highly preferredto use substantially pure oxygen as the oxidizer in order to preventexcess nitrogen from being introduced into the combustion process. Thesubstantially pure oxygen suitable as an oxidizer for the subjectinvention preferably has an oxygen purity of at least 95 mole percent(at least 0.950 mole fraction oxygen), more preferably of at least 97mole percent (at least 0.970 mole fraction oxygen), and most preferablyat least 99.5 mole percent (at least 0.995 mole fraction oxygen) forbest results.

[0026] A potential penalty associated with using substantially pureoxygen during the combustion process is a much higher adiabatic flametemperature, which may result in material failure of the fired heaterand/or coking of the radiant section tubes. Additionally, excessiveadiabatic flame temperature and radiant section temperatures may resultin excessive thermal NOx production. Accordingly, it is preferred thatthe adiabatic flame temperature ranges from about 2,500° F. to about4,000° F., more preferably from about 2,500° F. to about 3,500° F., andmost preferably, from about 3,000° F. to about 3,500° F., for bestresults. It is preferred that the radiant section temperature rangesfrom about 1,100° F. to about 2,200° F., and most preferably from about1,300° F. to about 1,800° F. for best results.

[0027] The volatility in adiabatic flame temperature is often due to theabsence of nitrogen that accompanies oxygen in air. Nitrogenconveniently absorbs heat from the combustion process, moderating theradiant section temperatures. Because of the absence of air-derivednitrogen during the combustion process, it is preferred that the subjectinvention recycles combustion flue gases to moderate these conditionsduring the combustion process.

[0028] The recycled flue gas suitable for the subject invention is theflue gases produced during the combustion of fuel in the fired heater.Flue gases produced during typical operation of fired heaters maycomprise water vapor, carbon dioxide, unburned fuel, nitrogen oxides,sulfur oxides, excess oxygen and nitrogen. Depending upon the fuel usedand other combustion conditions, the flue gas composition and theamounts of the individual components thereof may vary dramatically. Forinstance, the flue gas produced from combusting fuel in accordance withthe subject invention will primarily comprise water and carbon dioxidewith little or no objectionable pollutants, such as unburned fuel and/ornitrogen oxides.

[0029] The fired heaters suitable for the subject invention typicallycomprise a radiant section having one or more burners and radiantsection tubes that contain hydrocarbon process fluids, a convectionsection, and a flue gas stack section having at least one damper toassist in controlling the draft in the heater and/or directing the flowof the flue gas. Fuel combustion typically takes place in one or moreburners producing a flue gas that may be discharged into the atmosphereand/or recycled into the combustion process.

[0030] During the combustion process, the fuel and oxidizer aretypically injected into one or more burners. The oxidizer is preferablymixed with the recycled flue gas prior to its injection into the one ormore burners. However, the oxidizer may be injected directly into one ormore burners, into the fuel prior to injecting the fuel into one or moreburners or directly into the radiant section of the heater. It is alsopreferred to inject the oxidizer into the combustion process inquantities over stoichiometric oxygen requirements to insuresubstantially complete combustion of the fuel so that the flue gascomprises at least about one mole percent oxygen and preferably at leastabout three mole percent oxygen, resulting in substantially zerounburned hydrocarbons and CO being discharged into the atmosphere. Thecombustion of the fuel takes place in the presence of the oxidizer andrecycled flue gas in one or more burners or the radiant section of theheater.

[0031] Additionally, the combustion of the fuel can be conducted in sucha manner to eliminate the infiltration of tramp air from outside thefired heater, thereby reducing unnecessary NO_(x) production from thenitrogen present in air. It is generally preferred that the heater shellis sealed. In another embodiment, the heater may be operated such thatthe pressure of the radiant section is greater than the ambientpressure. When operating the heater such that the radiant section of theheater has a pressure greater than the ambient pressure, it is preferredthat the pressure of the radiant section of the heater is at least about0.05″ of H₂O, more preferably at least about 0.075″ of H₂O, and mostpreferably at least about 0.10″ of H₂O for best results. It is alsopreferred that the pressure of the radiant section of the heater is notgreater than about 0.25″ of H₂O, more preferably not greater than about0.20″ of H₂O. It is also preferred that the pressure of the radiantsection of the heater ranges from about 0.05″ of H₂O to about 0.10″ ofH₂O, and most preferably from about 0.05″ of H₂O to about 0.075″ of H₂Ofor best results. When combusting fuel in this manner it is preferred toassist the injection of the combustion oxidizer and/or recycled flue gaswith forced induction. This forced induction along with damper controlallows for maintaining the pressure of the radiant section at a pressuregreater than ambient pressure.

[0032] The combustion of the fuel in a fired heater as described hereinprimarily serves the purpose of heating hydrocarbon process fluids for avariety of refinery and/or petrochemical processes. In the case ofrefinery processes, fired heaters are widely used to heat hydrocarbonsin a variety of services, for example, catalytic reforming,isomerization, hydroprocessing, olefins manufacture, crude oil feed toan atmospheric tower, crude residuum from the atmospheric tower for feedto a vacuum tower, and the like. Perhaps the most severe service is theheating of feedstock to a delayed coker. While coke deposition can be aproblem in any refinery or petrochemical process furnace, because of thehigh temperatures employed and the residual nature of the cokerfeedstock, there is a pronounced tendency for the formation of cokedeposits on the inside wall of the radiant tubing. Regardless ofservice, the formation of coke deposits is not desirable. Coke depositscan lead to increased pressure in the tubes due to the restriction offlow, and to higher tube wall temperatures due to the insulative effectsof the coke deposits. Both higher process side pressure and temperaturelead to premature failure of the tubes. Furthermore, it is oftennecessary to periodically remove the tube from service and remove thecoke deposits by burning off the deposited coke by oxidation with air oranother oxidant that is passed through the tube at a high temperature.This periodic burn-off can result in severe thermal cycling, which alsoreduces the life of the tube.

[0033] One factor that has been identified as contributing to high cokeformation rates and high tube metal temperatures is the presence of heatflux imbalances. Heat-flux imbalances can be caused by many factors,including, but not limited to, furnace design and furnace operatingconditions, such as flame shape, adiabatic flame temperature,insufficient oxygen to burners and fuel gas composition.

[0034] In the case of water heating boilers, heat-flux imbalances is nota significant problem because boilers do not suffer the threat of cokeformation on the inside wall of the radiant section tubing. For firedheaters that heat hydrocarbon process fluids heat flux imbalances are asignificant problem. Therefore, it is favorable to maintain fired heaterflux rates for fired heaters that heat hydrocarbon process fluids fromabout 10,000 Btu/hr/sq ft. to about 12,000 Btu/hr/sq ft. to prevent cokeformation on the inside wall of the radiant tubing. Boilers, on theother hand, heat water, and therefore can have flux rates that aretypically from about 25,000 Btu/hr/sq ft. to about 50,000 Btu/hr/sq ft.As a result, boilers and heaters have separate operating regimes due todifferences in the substances each are designed to heat. Accordingly,the subject invention of operating a fired heater within the combustionregime as described herein specifically contemplates the heating ofhydrocarbon process fluids and solves the problem, among other problems,of heated hydrocarbon process fluid degradation and unnecessary cokeformation in radiant section tubes resulting from excessive flux rates.

[0035] The subject invention contemplates a relationship between theflue gas, the oxidizer, the fuel, and a numerical combustion regime(“the combustion regime”) to achieve beneficial results as hereindescribed, and is characterized with the aid of a mathematical equationor model which correlates the combustion regime as a function of therecycle ratio of the flue gas, the mole fraction of oxygen in theoxidizer, the temperature of the flue gas and the heating value of thefuel. Operating a fired heater within the combustion regime as describedherein allows for safe, economical and environmentally sound combustionof the fuel in the fired heater.

[0036] The mathematical model representing the combustion regime ispreferably${{4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500}},$

[0037] wherein w is the mole fraction of oxygen in the oxidizer, x isthe recycle ratio of the flue gas in moles flue gas/mole of oxidizer, yis the temperature in Fahrenheit of the flue gas as it is injected intothe combustion process, and z is the heating value of the fuel inBtu/scf LHV. In one embodiment, the combustion regime is represented by${4,000} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {3,000.}$

[0038] In another embodiment, the combustion regime is represented by${3,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500.}$

[0039] In yet another embodiment, the combustion regime is representedby${3,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {3,000.}$

[0040] In addition to operating the fired heater within the numericalrange representative of the combustion regime, it is also important toconsider the individual impact each combustion variable can have on theoperating conditions of the fired heater. For instance, changes in thefuel heating value and/or excessive flue gas recycling may jeopardizecontinuous combustion and flame stability within the fired heater.Reducing the mole fraction of oxygen in the oxidizer may result inincreased NOx production and/or reduced efficiency of the fired heater.Insufficient flue gas recycling and/or excessive recycled flue gastemperatures may result in excessive adiabatic flame and radiant sectiontemperatures.

[0041] Consequently, it is preferred that the oxidizer has a molefraction of at least 0.800 oxygen (at least 80 percent oxygen), and morepreferably at least 0.900 oxygen (at least 90 percent oxygen). It isalso preferred that the mole fraction of oxygen w in the oxidizer rangesfrom about 0.800 oxygen to about 0.995 oxygen (from about 80 percentoxygen to about 99.5 percent oxygen), more preferably from about 0.900oxygen to about 0.995 oxygen (from about 90 percent oxygen to about 99.5percent oxygen), and most preferably from about 0.950 oxygen to about0.995 oxygen (from about 95 percent oxygen to about 99.5 percent oxygen)for best results.

[0042] It is preferred that the recycle ratio x is at least 1 mole offlue gas/mole of oxidizer, more preferably at least 2.6 moles of fluegas/mole of oxidizer, and more preferably at least 3.6 moles of fluegas/mole of oxidizer. It is also preferred that recycle ratio x is notgreater than 11 moles of flue gas/mole of oxidizer, more preferably notgreater than 9 moles of flue gas/mole of oxidizer, and more preferablynot greater than 6.0 moles of flue gas/mole of oxidizer. However, it isalso preferred that the recycle ratio ranges from about 1.0 moles offlue gas/mole of oxidizer to about 8.0 moles of flue gas/mole ofoxidizer, and most preferably from about 2.6 moles of flue gas/mole ofoxidizer to about 6.0 moles of flue gas/mole of oxidizer for bestresults.

[0043] It is preferred that the temperature y of the recycled flue gasis not greater than 1,200° F., more preferably not greater than 800° F.,and more preferably not greater than 700° F. It is also preferred thatthe temperature y of the recycled flue gas is at least 300° F. and morepreferably at least 400° F. However, it is also preferred thattemperature y of the recycled flue gas ranges from about 300° F. toabout 1,200° F., more preferably from about 400° F. to about 800° F.,and most preferably from about 400° F. to about 700° F. for bestresults.

[0044] It is preferred that the heating value z of the fuel is at least200 Btu/scf LHV, and more preferably at least 300 Btu/scf LHV. It isalso preferred that the heating value z of the fuel is not greater than1,600 Btu/scf LHV, more preferably not greater than 1,200 Btu/scf, andmost preferably not greater than 900 BTU/scf LHV for best results.However, it is also preferred that the heating value z of the fuelranges from about 200 Btu/scf LHV to about 2,200 Btu/scf LHV, and mostpreferably from about 300 Btu/scf LHV to about 1,600 Btu/scf LHV forbest results.

[0045] It is relatively straightforward to consider and predict theimpact an individual combustion variable may have on the overallcombustion of fuel in a fired heater, and thereafter adjust thatindividual combustion variable accordingly. It becomes increasingly moredifficult as additional combustion variables must be considered becausetypically each combustion variable impacts the other as well as theoverall combustion process. Consequently, it is exceedingly problematicto predict the impact each combustion variable may have on each other aswell the overall combustion process, and thereafter operate the firedheater in a manner to maintain safe, economical and environmentallysound combustion of the fuel in the fired heater. However, the subjectinvention solves this problem by defining the relationship between theheating value of the fuel, the temperature and recycle ratio of the fluegas injected into the combustion process, the mole fraction of oxygen inthe oxidizer and the numerical combustion regime as defined herein.Through this relationship the subject invention allows for simpleoperation of the fired heater in accordance with the mathematical modelwithin the defined values of each combustion variable as well as thedefined value of the numerical combustion regime.

[0046] One of ordinary skill in the art will not only appreciate theindividual benefits realized from employing substantially pure oxygenand/or recycling flue gas to the combustion process, but will besurprised at the overall beneficial results achieved by the subjectinvention. To fully illustrate the breadth of the subject invention as awhole, reference is made to the FIG. 1.

[0047]FIG. 1 depicts operation of an fired heater in accordance with thesubject invention. As depicted in FIG. 1, heater 10 comprises one ormore burners 11, a radiant section 12, a bridgewall 13, a convectionsection 14, and a flue gas stack section 15. Typical operation of heater10 begins with the combustion of fuel supplied from fuel feed stream 1in the presence of substantially pure oxygen supplied from oxygen feedstream 2, producing a flue gas, which typically flows within heater 10from radiant section 12 to convection section 14 to flue gas stacksection 15. Bridgewall 13 is the point that separates the radiantsection 12 from convection section 14, and bridgewall 13 may serve as ameasuring point for the temperature and the pressure of the flue gasexiting the radiant section 12.

[0048] Flue gas stack section 15 will typically have damper 16 tocontrol the draft in the heater. In addition to controlling the draft inthe heater, one or more dampers may be used to direct the flow of theflue gas to the atmosphere and/or to the combustion process as flue gasrecycle stream 4. It is preferred that the damper is operated in such amanner to maintain the flow of the flue gas recycle stream 4 directedinto the combustion process at a recycle ratio ranging from about 1 moleflue gas/mole of oxidizer to about 8 moles flue gas/mole of oxidizer,and more preferably ranging from about 2.6 moles flue gas/mole ofoxidizer to about 6 moles flue gas/mole of oxidizer.

[0049] Preferably, the flue gas stack section 15 provides the source ofthe flue gas recycle stream 4, which is sent to one or more burners 11or radiant section 15 via flue gas recycle stream 4. However, convectionsection 14 can also provide the source of flue gas recycle stream 4.Flue gas recycle stream 4 can be located external of the heater, asillustrated in FIG. 1. In any event, it is preferred to measure the fluegas temperature, pressure and/or flow rate in the flue gas stack section15 and/or the flue gas recycle stream 4.

[0050] Typically, the flue gas recycle stream 4 exiting the flue gasstack section 15 is at a temperature ranging from about 500° F. to about800° F. If the flue recycle stream exceeds 700° F., it is preferred thatit undergoes cooling through recycle cooler 5, which can be of anyconventional type. If the flue gas recycle stream 4 undergoes recyclecooling through recycle cooler 5, it is preferred that the temperature,pressure and flow rate of flue gas recycle stream 4 are measureddownstream of recycle cooler 5. If necessary, the flow rate and pressureof the flue gas recycle stream 4 may be increased or variably controlledby induced draft fan 6 or by any conventional means suitable for thisprocess, including, but not limited to, increased natural draft effect.If flue gas recycle stream 4 has undergone a flow rate and/or pressurechange, it is preferred that the temperature and flow rate of flue gasrecycle stream 4 are measured downstream of said flow rate and/orpressure change. At least a portion of the flue gas recycle stream 4 maybe removed from the process via reject stream 7. The precise amount ofthe flue gas recycle stream 4 sent to reject stream 7 can be determinedas further described herein.

[0051] Reject stream 7 primarily consists of innocuous molecules, suchas water and carbon dioxide, and is almost completely devoid ofparticular environmental pollutants. As a result of the combustionprocess as described herein, reject stream 7 can be discharged into theatmosphere with relatively low environmental impact.

[0052] The flue gas recycle stream is preferably mixed with at least aportion of the oxidizer prior to its injection into one or more burners11. Referring again to FIG. 1, substantially pure oxygen is preferablyinjected via oxygen feed stream 2 directly into the flue recycle stream4. However, the substantially pure oxygen may be injected directly intoone or more burners 11, into fuel feed 1 or into the radiant section 12of the heater. The oxygen and flue recycle stream are thereafterpreferably injected into one or more burners 11 or the radiant section12 of the heater for fuel combustion.

[0053] The fuel is preferably injected into one or more burners 11 viafuel feed stream 1. However, prior to injecting the fuel into thecombustion process, it is preferred that the precise composition of thefuel is determined in order to calculate the heating value of the fuel.It is also preferred to determine flow rate of the fuel feed streamprior to injecting the fuel into the combustion process. Fuelcomposition determination can be accomplished by way of a chromatograph,a calorimeter or in any other conventional manner. However, it ispreferred that the composition of the fuel is continuously analyzed bysending at least a portion of the fuel though a sample cell of achromatograph 21, thereby continuously producing fuel composition datafor subsequent heating value calculations. The heating value of the fuelmay also be readily available or directly determined by a calorimetertest without the need for determining the composition of the fuel.

[0054] In a preferred embodiment, the subject invention alsocontemplates a feedforward combustion control system to facilitatecontinued operation of the fired heater within the combustion regime. Asdepicted in FIG. 1, the chromatograph 21 is operatively linked tomicroprocessor 23 via chromatograph output line 22. The chromatograph 21outputs fuel flow and/or composition data via chromatograph output line22 to the microprocessor 23. Using the fuel flow and/or compositiondata, microprocessor 23 can calculate the density, heating value andoxygen demand of the fuel. Microprocessors used for the subjectinvention may be of any variety available in the art and may be operatedsingularly or in tandem with one or more microprocessors. Other data,including, but not limited to, flue gas temperature, flue gascomposition, flue gas pressure and radiant section temperature, may beinput into the microprocessor 23 via additional input devices and inputlines. Such input devices may include, but are not limited to, akeyboard for manually inputting data.

[0055] The microprocessor 23 is programmed to calculate the heatingvalue of the fuel in fuel feed stream 1 based on the fuel compositiondata obtained from the chromatograph 23. Microprocessor 23 isoperatively linked to heater controller 25 via microprocessor outputline 24. Microprocessor 23 sends a signal to the heater controller 25instructing the heater controller 25 to perform one or moreinstructions.

[0056] The heater controller 25 operates a set of flow control valves.,As shown in FIG. 1, the flow control valves comprise fuel flow controlvalve 26, oxygen flow control valve 27, recycle flow control valve 28,and/or reject flow control valve 29. In one embodiment, flow controlvalves are actuated based on a signal from the microprocessor 23,thereby controlling the degree of flow of fuel feed stream 1, oxygenfeed stream 2 and/or flue gas recycle stream 4 injected into one or moreburners 11 and/or the degree of flow of the reject stream 7 info theatmosphere. The flow control valves may be actuated in any conventionalmanner, including, but not limited to pneumatically or hydraulically.Based on output data from the microprocessor 23, the flow of the fuelfeed stream 1, oxygen feed stream 2 and/or flue recycle stream 4 ispreferably controlled by heater controller 25 to continuously maintainoperation of the fired heater within the combustion regime in accordancewith the present invention. This feedforward combustion control schemefacilitates operation of the fired heater within the combustion regimethat may not otherwise be possible in the event of a significant changein any input combustion condition, such as changes in fuel compositionor flow rate, changes in the oxidizer composition or flow rate and/orchanges in flue gas recycle ratios.

[0057] As a result, the present invention provides for efficientoperation of a fired heater by eliminating heat flux imbalances andinstable flame patterns, thereby resulting in substantial cost savingsthrough: (1) the elimination of unnecessary fired heater downtime due tothe coking of radiant section tubes; and (2) unnecessary consumption ofexpensive oxygen and fuel.

[0058] The present invention also provides for substantial environmentalbenefits and safety benefits to person and property by avoidingdangerous operating conditions, which could result in: (1) the dischargeof environmental and toxic pollutants, such as NO_(x), CO and unburnedhydrocarbons, into the atmosphere; and (2) possible material failure ofthe fired heater due to excessive radiant section temperatures.

[0059] The present invention also provides for a simple design optionfor the combustion of fuel in a fired heater, facilitatingimplementation of the subject invention for existing fired heatersutilized throughout the refining and petrochemical industry.

[0060] Although the present invention has been described withparticularity and detail, the following examples provide furtherillustration of the invention and are understood to not limit the scopeof the invention.

[0061] The following examples represent detailed computer simulations ofcombusting fuel in a sealed fired heater for heating crude oil,operating substantially in accordance with the present invention andFIG. 1. FIG. 2 graphically depicts the combustion regime in accordancewith the examples where the mole fraction of oxygen in the oxidizer is0.995. In conducting the simulations and as illustrated in the followingtables, a variety of process parameters that affect the degree ofsafety, efficiency and environmental impact of combusting fuel in afired heater were considered during each test run.

EXAMPLE 1

[0062] This simulation is conducted in accordance with the mathematicalrepresentation of the combustion regime of${4,500} \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq {2,500.}$

[0063] In this simulation, the heating values of the fuel range from 300Btu/scf LHV to 900 Btu/scf LHV. An oxidizer with a mole fraction of0.995 oxygen (99.5 percent oxygen) is injected into the combustionprocess over stoichiometric oxygen requirements so that the flue gascomprises about 3 mole percent oxygen. The flue gas recycle ratios rangefrom 2.901 to 5.285 moles flue gas/mole of oxidizer. The recycled fluegas temperature ranges from 400° F. to 700° F. The exact combustionparameters and results for each test run are set forth in Table 1,wherein f represents a value within the combustion regime. TABLE 1 FuelO2 in Heating Flue Flue Flue Value Excessive O2 in Gas Gas Gas (Btu/Oxidizer Oxidizer (Wet) Temp. Recycle Run scf) Fract Fract Fract ° F.Ratio f 1 300 .0598 .995 .0300 400 3.562 3500 2 300 .0598 .995 .0300 4004.120 3250 3 300 .0598 .995 .0300 400 4.793 3000 4 300 .0598 .995 .0300700 3.855 3500 5 300 .0598 .995 .0300 700 4.496 3250 6 300 .0598 .995.0300 700 5.285 3000 7 900 .0466 .995 .0300 400 2.901 3500 8 900 .0466.995 .0300 400 3.340 3250 9 900 .0466 .995 .0300 400 3.870 3000 10 900.0466 .995 .0300 700 3.141 3500 11 900 .0466 .995 .0300 700 3.647 325012 900 .0466 .995 .0300 700 4.269 3000

[0064] In this simulation, it is projected that the flue gas compositionfor each test run yields substantially zero NO_(x), CO, or unburnedhydrocarbons in the flue gas. In all instances, the heat flux isprojected to be well within the limits of safe and efficient operationof the fired heater, resulting in an expected reduction in the downtimeof the fired heater due to material failure and/or coking of the radiantsection tubes.

EXAMPLE 2

[0065] The simulation is conducted in accordance with the mathematicalrepresentation of the combustion regime of$4,{500 \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq 2},500.$

[0066] In this simulation, the heating value of the fuel is 700 Btu/scfLHV. An oxidizer comprising 99.5 mole percent oxygen (substantially pureoxygen) is injected into the combustion process over stoichiometricoxygen requirements so that the flue gas comprises about 3 mole percentoxygen. The flue gas is recycled into the combustion process at a rateof 1.0 mole flue gas/mole of oxidizer at a temperature of 700° F. Theexact combustion parameters and results for the test run are set forthin Table 2, wherein f represents a value outside the combustion regime.TABLE 2 Fuel O2 in Heating Flue Flue Flue Value Excessive O2 in Gas GasGas (Btu/ Oxidizer Oxidizer (Wet) Temp. Recycle Run scf) Fract FractFract ° F. Ratio f 1 900 .0466 .995 .0300 700 1.0 4928

[0067] In this simulation, it is projected that premature deteriorationof the burner and adjacent structure (e.g. refractory lining) will occurfrom long term exposure to excessive adiabatic flame temperatures. It isalso projected that radiant section tubes may ultimately suffer thermalstress ruptures resulting from extensive local overheating or hot spotson the radiant section tubes. In both cases, catastrophic failure of thefired heater may occur resulting in a substantial safety hazard toperson and property.

[0068] Although embodiments of this invention have been shown anddescribed, it is to be understood that various modifications andsubstitutions, as well as rearrangement of parts and equipment, can bemade by those skilled in the art without departing from the novel spiritand the scope of this invention.

That which is claimed is:
 1. A method for combusting fuel in a firedheater having one or more burners, the method comprising: (a) combustingat least a portion of said fuel in the presence of an oxidizer in saidburners, generating heat and producing a flue gas; and (b) recycling atleast a portion of said flue gas to said burners; wherein step (a) andstep (b) are conducted in a manner such that combusting takes place in acombustion regime represented by$4,{500 \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq 2},{500;}$

wherein w represents a mole fraction of oxygen in the oxidizer; whereinx represents a recycle ratio of said flue as measured in moles of fluegas/mole of oxidizer; wherein y represents a temperature of the fluegas, in Fahrenheit, recycled to said burners; and wherein z represents aheating value of said fuel in Btu/scf LHV.
 2. The method of claim 1,further comprising the step of heating a hydrocarbon process fluid fromthe heat generated in step (a).
 3. The method of claim 1, wherein1.0≦x≦8.0.
 4. The method of claim 1, wherein 2.6≦x≦6.0.
 5. The method ofclaim 2, wherein 1.0≦x≦8.0.
 6. The method of claim 2, wherein 2.6≦x≦6.0.7. The method of claim 1, wherein said oxidizer is oxygen-enriched air.8. The method of claim 1, wherein said oxidizer is substantially pureoxygen.
 9. The method of claim 2, wherein said oxidizer isoxygen-enriched air.
 10. The method of claim 2, wherein said oxidizer issubstantially pure oxygen.
 11. A method for combusting fuel in a firedheater having one or more burners, the method comprising: (a) combustingat least a portion of said fuel in the presence of an oxidizer in saidburners producing a flue gas; and (b) recycling at least a portion ofsaid flue gas to said burners; wherein step (a) and step (b) areconducted in a manner such that combusting takes place in a combustionregime represented by$4,{500 \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq 2},{500;}$

wherein w represents a mole fraction of oxygen in the oxidizer, wherein0.900≦w≦0.995; wherein x represents a recycle ratio in moles fluegas/mole of oxidizer, wherein 1.0≦x≦8.0; wherein y represents atemperature, in Fahrenheit, of the flue gas recycled to said burnerswherein 300° F.≦y≦1,200° F.; and wherein z represents a heating value inBtu/scf LHV of said fuel wherein 100 Btu/scf≦z≦2,200 Btu/scf.
 12. Themethod of claim 11, wherein said combustion regime is represented by$3,{500 \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq 2},500.$


13. The method of claim 11, wherein the fired heater has a heat fluxrate ranging from about 10,000 Btu/hr/sq ft. to about 12,000 Btu/hr/sqft.
 14. The method of claim 12, wherein the fired heater has a heat fluxrate ranging from about 10,000 Btu/hr/sq ft. to about 12,000 Btu/hr/sqft.
 15. A method for operating a fired heater having a radiant section,the method comprising: (a) combusting a fuel in the presence ofsubstantially pure oxygen in one or more burners of said fired heaterproducing a flue gas; and (b) recycling at least a portion of said fluegas to one or more burners; wherein the radiant section operates at apressure greater than ambient pressure.
 16. The method of claim 15,wherein the radiant section operates at a pressure of at least about0.05″ of H₂O.
 17. The method of claim 15, wherein the radiant sectionoperates at a pressure from at least about 0.05″ of H₂O to about 0.10″of H₂O.
 18. A method for monitoring and controlling the combustion offuel in a fired heater comprising a fuel feed stream, an oxygen feedstream and a recycle feed stream, the method comprising the steps of:(a) monitoring said fuel, producing fuel composition data; (b) feedingat least a portion of said fuel to said fired heater; (c) combustingsaid fuel in the presence of substantially pure oxygen in one or moreburners of said fired heater producing a flue gas having a temperaturey, as measured in Fahrenheit; and (d) recycling at least a portion ofsaid flue gas at a recycling ratio x, as measured in moles of fluegas/mole of oxygen, to the fired heater; wherein said fuel,substantially pure oxygen, and flue gas are fed into said fired heaterin at least stoichiometric amounts wherein combusting takes place in acombustion regime represented by$4,{500 \geq {7554.8 - {933.72x} + {64.960x^{2}} + {{.47705}y} - {{.55680}z} - \frac{1579.2}{w}} \geq 2},500.$


19. The method of claim 18, wherein said fuel is continuously monitoredby a chromatograph.
 20. The method of claim 19, wherein saidchromatograph is operatively linked to a microprocessor.
 21. The methodof claim 20, wherein said fuel composition data is sent from saidchromatograph to said microprocessor wherein said heating value of saidfuel is determined by said microprocessor based on said fuel compositiondata.
 22. The method of claim 21, wherein said microprocessor isoperatively linked to a heater controller wherein said heater controlleroperates a fuel flow control valve, an oxygen flow control valve, and arecycle flow control valve based on one or more signals from themicroprocessor, thereby controlling the amount of fuel, substantiallypure oxygen and flue gas fed into the fired heater.